Tensile Testing Grips - A Guide to Buying the Correct Fixtures

ADMET has many years of experience in the material testing industry and this has shown the significance of equipping test machines with the exact grips and fixtures.

A machine can only be exploited to its full capacity if the right fixtures or grips are used. With all the available options, selecting the correct accessories for a testing application can be more complicated than one thinks.

Grips and fixtures are mainly categorized by two key factors: the kind of testing that they will be used for and their capacity. Overloading the grips may damage not only the grips but also the machine frame and the load cell, so it is very important to avoid overloading. This article will focus on grips employed for tensile testing and guide customers through the steps involved in determining the appropriate tensile grips for their testing.

Common Issues Experienced with Grips

Specimen slippage from the grip faces

Specimen breaking at the grip faces

Not being able to tightly grip samples because of their non-standard shapes or dimensions

Specimen Slippage

Specimen slippage from the grip faces may occur not only due the material characteristics of samples, but also due to several reasons associated with the grips selected:

The clamping force is not high enough to grip the samples

In order to obtain higher clamping forces, customers need to switch to pneumatic grips. Almost all ADMET grips are provided with both the manual and the pneumatic clamping mechanism.

The surface area of the grip jaw faces may not be sufficiently covering a large surface area to hold the samples

Most tensile testing standards suggest gripping at least 3/4th of the specimen surface area in order to ensure that the specimen is firmly gripped and is set to undergo a tensile test. ADMET grip jaws are available in different sizes.

The jaw surface may not be the correct type for the sample material of the customer

To select the most suitable jaws, customers should discuss the specifics of their samples and testing application with the company’s Sales Engineers.

Specimen Breaking at the Grip Faces

Instead of slipping from the grips, certain specimens may break right at the grip faces. It is necessary to monitor the behavior of the sample during the tests and observe how the specimen breaks. The recorded breaking strength will not accurately represent the actual breaking strength if the break is at the line of the jaw faces. There are a few ways to handle specimen breakage:

When using pneumatic grips, customers should adjust the testing pressure in order to find the perfect clamping force that would be enough to avoid slippage yet that would not be extreme and result in specimen break at grip faces.

When using manual vise grips, customers must ensure to check that the manual clamping force on the specimen is not very high.

If pneumatic or manual vise grip solutions do not work, customers can utilize grips with different designs such as eccentric roller or webbing grips. These grips are designed in such a way that there is no contact between the square grip jaws and the sample.

Another reason for specimen break at grips might be because of the serrations of serrated jaws damaging the specimen. In such situations, customers can switch to another jaw type or select a customized serrated jaw set.

Difficult or Awkward Sized/Shaped Samples

What if the samples are too small or too large and have a non-standard shape that it is almost impossible to find the right grips? This might also be an issue if customers are interested in testing the end product, not just the material with which the end product is made. Very often, the solution is to have a custom grip engineered to suit the customers’ testing requirements or look into non-standard grips which are not always listed in product catalogs.

One example is a fixture that is used for holding needles. The ADMET’s Engineers have developed the Luer lock to adapter fixture, as shown in the picture below, for a quick solution.

Custom Luer Lock test fixture.

Five Things to Know Before Buying Tensile Testing Grips

Keeping those items in mind, users can avoid the most common issues associated with tensile grips by being conscious of five key factors in choosing the correct grips for their application.

Vise grips are the most common tensile grips and ADMET offers these grips with one or two t-handles.

Figure 1. Manual Vise Grip diagram.

Particular tensile grips may be limited to the opening width of the jaws or limited to a specific capacity due to their design specifications. For instance, manual vise grips can test samples up to 50 kN. Pneumatic grips can go up to 30 kN, but when the capacity increases, the opening width of the grip jaws decreases.

Wedge grips are often used to test higher capacity samples such as metals. They are also used with ASTM E8 metals testing and supplied with optional alignment tools in order to ensure axiality of the applied loads.

Scissor grips provide a large opening width together with wide jaws, and hence they can be used with samples with non-standard shapes, such as plastic belts.

Figure 6. Scissor grips.

O-ring tensile testing fixture, loop testing grips and hook grips are examples of non-standard solutions that can be a better choice for customers’ materials.

Figure 7. Hook grips.

Figure 8. O-Ring tensile grips.

Item #2 – Grip Faces (Jaws)

The same grip can be employed for testing several samples just by changing the grip face type. Without knowing the specifications of the sample material, it would be difficult to recommend a grip face. Certain ASTM standards will contain a recommended grip face, but most will leave it to the user to decide based on the dimensions and characteristics of the specimen material.

Grip Face

Description

Blank

Blank, smooth jaws are steel faced jaw surfaces without any coating. They are often used with foils and film.

Rubber

Rubber jaws come with rubber coatings and are recommended for paper specimen and biomaterials.

Wave jaws are commonly used with flexible materials that are held firmly between the 5 mm waves.

V-Jaws

V-jaws are used with round samples. Customers must ensure to check the diameter specification for each v-jaw prior to making their final decision. Besides, v-jaws may not always be the most suitable for hollow tube samples. Tube samples can be tested with flat jaws if a pin is inserted through them.

Line Contact

Line contact jaws are made up of a smooth, nickel-plated surface and a 3 mm line in the center of the jaw. Line jaws deliver a better grip for sheet materials and plastic films.

In addition, ADMET provides a quick change system for all types of jaw surface mentioned above. Users who are interested in quickly changing from one jaw type to another can use the jaws with quick change carriers.

Figure 9. Quick change grip faces.

Item #3 – Grip Capacity

Tensile grips are employed to achieve results on the tensile properties of customers’ samples such as yield, tensile modulus, elongation, peak load and tensile strength.

Different materials demonstrate different material characteristics. Before selecting grips, customers must be confident of the maximum force that will be needed to test their samples.

Hydraulic grips are the highest capacity and can go up to 3,000 kN capacity.

Pneumatic grips are employed with plastic and rubber samples as they can only go up to 30 kN.

Wedge grips work with materials that need high capacities such as composites and metals and can go up to 500 kN.

Webbing grips have capacities ranging from 5 kN to 200 kN. Since the capacity of the webbing grips increases, the roll diameter and the accommodated sample width also increase.

Rope grips can go up to 50 kN.

Thread grips are commonly used with fishnets, yarns, cords and wires, and they are offered in capacities ranging from 200 N to 5 kN.

Pinching grips are used together with low-capacity samples such as biomaterials and electronic components. The pinching grips’ maximum force can go up to 2 kN.

Manual vise grips are flexible tensile grips and can go up to 50 kN.

Item #4 – Specimen Material and Geometry and their Impact

The end product and its internal component materials with varying features can be tested for tensile properties using a universal testing machine.

Common materials tested in tension include textiles, paper, metals, plastics and elastomers, composites, biomaterials and adhesives. At first, each material is prepared for testing in a particular geometry that is typically defined in the testing standards. Common specimen geometries include cylindrical or flat dog-bone geometries such as bars and tubes.

Sample preparation may be performed by die cutting or machining. ADMET provides dies conforming to particular testing standards. Sample preparation is essential and usually specific. One example is ASTM D638, which explains five different specimen dimensions that have to be selected based on the characteristics of the material that needs to be tested.

Figure 10. ASTM D638 die specifications.

Very often, hard specimens such as composites, metals and certain plastics are difficult to hold properly as slippage and premature breakage can be common. Therefore, grips with high clamping forces, such as wedge grips and hydraulic grips are recommended.

On the contrary, elastomers and rubbers are more easily held in a range of grip designs such as wedge grips, pneumatic grips, manual vise grips or eccentric roller designs. The easy-to-use and most common grips are pneumatic or manual vise grips with rubber coated or serrated jaw faces.

In addition to recommending the test method, particular testing standards indicate a fixture design to be made specifically for testing in accordance with that standard. Examples of tensile fixtures that are designed to specific standards include ASTM D3039, ASTM D1414 and ASTM C297.

Figure 11. ASTM C297 custom test fixture.

Item #5 – Adapting/Mounting Grips to Testing Machines

The final item on the list is attaching the grips on the testing machine. Since grips are not linked to the electronics of customers’ universal testing machine, they can be used together with any make and model. Usually, ADMET tensile testing grips are mounted on Tinius Olsen, MTS and Instron frames using adapters.

Adapters can be provided with customers’ grips of choice of if customers would like to machine them in-house, the company’s Sales Engineers can provide drawings to match the precise dimensions required.

Figure 12. Male end adapter for mounting grips.

Conclusion

To determine the best suited grips for tests, customers can let ADMET’s Sales Engineers know the maximum force capacities required to test their samples, shapes, dimensions, sample material and elongation properties. The Engineers will then discuss the options with the customers, and, if required, the team can even test their samples to select the right grips for their testing requirements.

This information has been sourced, reviewed and adapted from materials provided by Admet, Inc. - Materials Testing Equipment.